1. Field of the Invention
The present invention relates to a road-surface condition estimating device that estimates a road-surface condition with high accuracy on the basis of a relationship between a road-surface friction coefficient and a slip rate of a vehicle wheel.
2. Description of the Related Art
In recent years, there have been proposed and put in practical use various control technologies for vehicles, such as traction control, braking-force control, and torque-distribution control technologies. These technologies generally utilize a road-surface friction coefficient, which indicates a road-surface condition, for the calculation or the correction of required control parameters. In order to perform the control properly, it is necessary to estimate a road-surface friction coefficient with high accuracy.
For example, Japanese Unexamined Patent Application Publication No. 7-159308 discloses a technology for estimating a road-surface friction coefficient. Specifically, the estimation is implemented by calculating a difference in speed between front and rear wheels, integrating the differences in speed sequentially at predetermined intervals to obtain an integrated value, calculating an average gradient of the integrated value within a predetermined time period, determining whether it is an appropriate time for estimating a road-surface friction coefficient from the calculated average gradient within the predetermined time period, and estimating a road-surface friction coefficient from the average gradient of the finely variable wheel speeds within the predetermined time period if the determination result shows that it is an appropriate time for the estimation of a road-surface friction coefficient.
In the technology disclosed in Japanese Unexamined Patent Application Publication No. 7-159308, the relationship between a road-surface friction coefficient μ and a slip rate λ of the wheel is as shown in, for example, FIG. 4. Specifically, this technology utilizes the characteristic in which when the slip rate λ becomes λs and an amount of change in road-surface friction coefficient with respect to an amount of change in slip rate (dμ/dλ) becomes zero, a value of a road-surface friction coefficient μ in that state finally becomes the road-surface friction coefficient μ. This implies that the value of the road-surface friction coefficient μ can only be estimated when the wheel is near the grip limit. Since the estimation of the road-surface friction coefficient μ is not possible in other running ranges being out of when the wheel is near the grip limit this technology is problematic in terms of low versatility. In most vehicle controls, it is necessary to ensure stability of the vehicle by properly controlling the vehicle behavior before the wheel reaches the grip limit, and therefore, it is difficult to apply the technology to such vehicle controls. Moreover, although the estimation of a road-surface friction coefficient μ in the Japanese Unexamined Patent Application Publication No. 7-159308 is implemented based on a theory that the product of the road-surface friction coefficient μ and the ground load of the wheel is equal to the total driving force, if the vehicle is on a sloped road, it is not possible to obtain a road-surface friction coefficient μ with high accuracy unless errors caused by the slope are taken into account.